If you've been having trouble accessing some of your favorite websites, or accessing the internet at all, you're not alone. It appears that Comcast's Xfinity internet service is experiencing nationwide outages that are affecting large swaths of its customer base.

Some of the major cities affected by the outage include Chicago, Seattle, Houston, Boston, New York City and San Francisco -- these represent some of the most densely-populated cities in America. DownDetector.com has a live look at Comcast's current connectivity woes, which you can see here:

For its part, Comcast has provided an update on the situation, but doesn't currently have an ETA for when service will be fully restored:

Some customers are having issues with their XFINITY Internet service. We apologize & appreciate your patience while we work to fix.

Comcast customer in the meantime have flocked to DownDetector to express their frustrations with the slow or non-existent service. "LOL trying to download something, speeds are ranging from 2kbps to 131kbps," wrote Iona Davidson. "Not sure what they are supposed to be, but.... I'm about fed up with Comcast."

"Having slow Internet connection why in the heck am I paying almost 400.00 a month for services only to have crappy service.. Fix this crap Xfinity..." wrote Tanya Loza. Rather humorously, Todd Word wrote, "The Xfinity Status Center claims, ‘Everything looks good on our end. We don’t see any outages at the moment.’ So, mental note for the future, I guess that can't be trusted."

But before everyone starts pointing their fingers solely at Comcast, it appears that the outage is a result of an issue with Level 3 Communications, which is the nation's third largest fiber optic carrier. DownDector shows that Level 3, which is a Tier 1 backhaul provider for ISPs, began experiencing problems around the same time that the Comcast outages started.

This recent tweet from Comcast seems to confirm that notion:

UPDATE: our teams continue to monitor an external network issue. We apologize for the inconvenience & will provide updates as we learn more.

So, if you're among the millions that are likely without service or are simply dragging along at a snail's pace, at least take comfort in the fact that the situation is in the process of being rectified.